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Yes, you can do this with OBS. The virtual webcam feature can be added as a plugin. And chroma-keying is a built-in feature. I use a Mac, and I've been toying with this a bit over the last weeks (e.g. https://davidbieber.com/snippets/2020-04-09-exploring-obs/ )
It's super easy...if you have a suitable chromakey backdrop and decent lighting on it. What I'd love is a plugin that replicates (or at least approximates) some of the "virtual chromakey" features of Zoom et al. I don't currently have access to a good chromakey backdrop and it really is much easier to do it well with a bright green matte screen behind you. It ain't important, but it would sure be nice!
There are green screen plugins for that. You could chain Snapcamera's virtual cam output into OBS and use OBS 's chromakey filter to underlay whatever other content you like. Y I u can then output OBS to another virtual cam output for use in Zoom or wherever else via the Virtualcam plugin for OBS.
It'd be really nice if someone would make a nice OpenCV virtualcam output so one could drive all this from a (relatively) simple Python script.